A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Position measurement systems are used for position measurement of different parts of a known lithographic apparatus. In particular, for objects which have to be positioned with high accuracy, such as the patterning device support and the substrate support of a scanning type lithographic apparatus, it is desirable to determine the position with high accuracy.
Both laser interferometric as encoder-type position measurement systems are applied in position measurement systems of known lithographic apparatus. As prior art interferometric systems may be more susceptible to environmental influences, a general preference is given to encoder type measurement systems including one or more encoder heads on a first object and a grid or grating on another object.
In one application of a position measurement of a patterning device stage system, the encoder heads are arranged on a reference object, while the grid or grating is arranged on the movable object of which the position is to be determined. As a result, relative long grid plates are used on the movable object to cover the whole range of movement. This prevents direct patterning device position measurement, i.e. direct determination of the position of the patterning device using a grid or grating provided on the patterning device itself. As a consequence of the presence of the relative large grid plate, the moving mass and dynamic performance of patterning device support may be negatively influenced. Also, the thermal expansion of the moving grid plate should be controlled in order to obtain reliable measurements.
Another alternative encoder-type position measurement system includes encoder heads that are mounted on the substrate support stage system, and the grid plate is mounted on the stationary frame. As a result, the data transfer and power cables to the encoder heads are arranged on the moving substrate support. In this arrangement, fibers move at high velocities and accelerations, and deteriorate stage dynamics and conditioning, as well as reliability, especially for targeted high accelerations.